Reproduction in Plants and Animals

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Reproduction is the biological process by which organisms produce offspring, ensuring the continuation of their species. It can be asexual (no gametes, no fertilisation) or sexual (gametes fuse).

Asexual Reproduction:

• One parent only
• No gametes or fertilisation
• Offspring are genetically identical clones (except mutations)
• Methods: Binary fission (bacteria), budding (yeast, Hydra), spore formation (fungi, ferns), vegetative propagation (plants), fragmentation (planarian worms)

Sexual Reproduction:

• Two parents usually
• Gametes (sex cells) form by meiosis
• Fertilisation: Male and female gametes fuse
• Offspring genetically unique (variation)
• Zygote: Fertilised egg cell

Sexual Reproduction in Plants:

Flower Structure:

• Stamens (male): Anther (produces pollen) + Filament
• Carpels/Pistil (female): Stigma (receives pollen) + Style + Ovary (contains ovules)
• Other parts: Petals, sepals, nectary

Pollination:

• Self-pollination: Pollen lands on stigma of SAME flower (or same plant)
• Cross-pollination: Pollen transferred to DIFFERENT flower (usually by wind, insects, birds)
• Agents of pollination: Wind (anemophily), Insects (entomophily), Birds (ornithophily), Water (hydrophily)

Double Fertilisation (Angiosperms):

1. Pollen grain germinates on stigma → pollen tube grows down style
2. Tube nucleus leads; generative nucleus divides → 2 sperm nuclei
3. One sperm + egg cell → zygote (2n) → embryo
4. Other sperm + 2 polar nuclei → triploid endosperm (3n) → food storage
5. Ovule → seed; ovary → fruit

Sexual Reproduction in Humans:

Male Reproductive System:

• Testes: Produce sperm (spermatogenesis) and testosterone
• Epididymis: Sperm maturation and storage
• Vas deferens: Transports sperm to urethra
• Seminal vesicles + Prostate gland: Produce seminal fluid (nourishes and transports sperm)
• Urethra: Carries sperm out of body
• Penis: Deposits sperm in female vagina

Female Reproductive System:

• Ovaries: Produce eggs (oogenesis) and oestrogen/progesterone
• Oviducts (Fallopian tubes): Site of fertilisation; cilia move egg toward uterus
• Uterus (womb): Implantation of embryo; site of pregnancy
• Cervix: Lower part of uterus; narrow opening
• Vagina: Receives penis; birth canal

⚡ WAEC Tip: In human reproduction questions, be precise with terminology. Sperm are produced in the seminiferous tubules (NOT the epididymis). Eggs are released from the ovary (NOT the Fallopian tube — the tube just collects the egg).

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding.

Gametogenesis:

Spermatogenesis (in testes): Spermatogonium (2n) → Primary spermatocyte (2n) → Meiosis I → 2 secondary spermatocytes (n) → Meiosis II → 4 spermatids (n) → Spermatozoa (n)

Each spermatid transforms into a sperm with: head (nucleus + acrosome), midpiece (mitochondria), tail (flagellum).

Oogenesis (in ovaries): Oogonium (2n) → Primary oocyte (2n) → Meiosis I → Secondary oocyte (n) + First polar body (n) → Meiosis II → Ovum (n) + Second polar body (n)

Key differences:

• Meiosis I completes BEFORE birth (females are born with all primary oocytes)
• Meiosis II completes ONLY if fertilisation occurs
• Unequal cytoplasm division → one large ovum, small polar bodies

Menstrual Cycle (28 days):

Phase	Days	Events
Menstruation	1-5	Uterine lining sheds (if no pregnancy)
Follicular phase	6-14	FSH stimulates follicle growth; oestrogen rises; endometrium rebuilds
Ovulation	14	LH surge → egg released from ovary
Luteal phase	15-28	Progesterone maintains endometrium; if no pregnancy, corpus luteum degenerates

Hormonal Control:

• FSH (Follicle Stimulating Hormone): Stimulates follicle development (pituitary)
• LH (Luteinising Hormone): Triggers ovulation and corpus luteum formation (pituitary)
• Oestrogen: Thickens endometrium; inhibits FSH (positive feedback before ovulation, negative after)
• Progesterone: Maintains endometrium for implantation; inhibits FSH and LH

Pregnancy and Development:

Fertilisation (usually in oviduct):

1. Sperm capacitation (final maturation in female tract)
2. Acrosome reaction → releases enzymes to penetrate egg’s zona pellucida
3. Sperm binds to ZP3 receptors on zona pellucida
4. Cortical reaction → zona reaction (prevents polyspermy)
5. Sperm enters ooplasm
6. Second meiosis II completes → female pronucleus forms
7. Male and female pronuclei fuse → zygote

Embryonic Development:

• Zygote → Mitotic divisions → Morula → Blastocyst → Implantation in uterus
• Blastocyst: Inner cell mass (embryo) + trophoblast (placenta)
• Gastrulation: Three germ layers form (ectoderm, mesoderm, endoderm)
• Organogenesis: Germ layers differentiate into organs

Asexual Reproduction in Plants:

Vegetative Propagation:

• Runners/Stolons: Strawberry — horizontal stems above ground
• Rhizomes: Ginger — horizontal stems below ground
• Tubers: Irish potato — modified underground stems with buds (“eyes”)
• Bulbs: Onion, garlic — underground shoots with fleshy leaves
• Corms: Cocoyam, yam — swollen underground stems
• Offsets: Daughter plants from parent plant (e.g., plantain)

Artificial Propagation:

• Cuttings: Stem sections placed in soil → roots develop
• Grafting: Scion (desired variety) attached to rootstock of another plant
• Budding: Single bud grafted onto rootstock (e.g., citrus)
• Layering: Stem bent to ground, covered with soil → roots develop

Advantages of Vegetative Propagation:

• Faster than growing from seeds
• Offspring genetically identical (desirable characteristics preserved)
• Some plants have lost ability to produce viable seeds

⚡ Common Student Mistakes: Students confuse gamete production in plants vs animals. In plants, pollen = male gametophyte (not just sperm carrier). Also confuse the functions of LH and FSH. Confusion between oestrogen’s dual role (positive feedback before ovulation, negative after). Confuse identical twins (same zygote splits) with fraternal twins (two separate zygotes).

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for thorough preparation.

Sexual Reproduction in Plants — Pollination and Fertilisation:

Pollen Development:

• Anther contains pollen mother cells (2n)
• Meiosis → 4 haploid microspores (n)
• Each microspore → pollen grain (male gametophyte)
• Pollen grain has 2 nuclei: tube nucleus + generative nucleus

Ovule Development:

• Ovary contains ovule mother cells (2n)
• Meiosis → 4 megaspores (n) — usually 3 degenerate
• Surviving megaspore → embryo sac (female gametophyte)
• Embryo sac has 8 nuclei: egg (n) + 2 synergids + 3 antipodals + 2 polar nuclei

Pollination Mechanisms:

Wind pollination (anemophily):

• Light, abundant pollen
• Large, feathery stigmas
• No nectar, no petals
• Examples: grasses, maize, many trees

Insect pollination (entomophily):

• Sticky, heavy pollen
• Brightly coloured, scented flowers
• Nectar guides (patterns visible in UV light)
• Nectar rewards
• Examples: Roses, sunflowers, citrus

Outbreeding Mechanisms (prevents self-pollination):

• Stamens and carpels mature at different times (protandry, protogyny)
• Self-incompatibility genes (S-alleles) — pollen tube rejected
• Dioecious plants (separate male and female plants)

Fruit and Seed Formation:

Ovary Part	Becomes
Ovule	Seed
Ovary wall	Fruit flesh
integuments	Seed coat
Zygote	Embryo
Polar nuclei + sperm	Endosperm
Ovary	Fruit

Types of Fruits:

True fruits: Develop from ovary (tomato, mango, maize) False fruits: Develop from other parts (apple — receptacle, strawberry — ovary)

Dehiscent fruits (split open):

• Follicle: One seam (poppy)
• Legume: Two seams (beans, peas)
• Silique: Two seams, seeds on central placenta (mustard)
• Capsule: Multiple seams/pores (cotton, okra)

Indehiscent fruits (don’t split):

• Achene: Single seed, free (sunflower, maize)
• Caryopsis: Like achene, seed fused to fruit wall (all cereals)
• Nut: Hard fruit wall (cashew)
• Drupe: Stone fruit with pit ( mango, oil palm)

Human Reproduction — Fertility and Contraception:

Contraceptive Methods:

Method	How it Works	Failure Rate
Condom (male)	Barrier — prevents sperm entry	~13%
Pill (combined oestrogen + progestin)	Inhibits ovulation	~7%
IUD	Copper interferes with sperm; progestin thickens cervical mucus	<1%
Tubal ligation	Surgical — Fallopian tubes cut/tied	<1%
Vasectomy	Surgical — vas deferens cut	<1%
Natural family planning	Monitor cycle → avoid fertile days	~24%

Infertility Treatments:

• IVF (In Vitro Fertilisation): Egg and sperm combined in lab → embryo transferred to uterus
• ICSI (Intracytoplasmic Sperm Injection): Single sperm injected directly into egg
• Donor eggs/sperm: For genetic disorders or same-sex couples
• Surrogacy: Another woman carries the pregnancy

Sexually Transmitted Infections (STIs):

Infection	Cause	Symptoms	Treatment
Gonorrhoea	Bacteria (Neisseria)	Discharge, pain	Antibiotics
Syphilis	Bacteria (Treponema)	Sores, rash	Penicillin
Chlamydia	Bacteria	Often no symptoms	Antibiotics
HIV/AIDS	Virus	Immune deficiency	Antiretroviral drugs
HPV	Virus	Genital warts, cervical cancer	No cure; vaccination
Herpes	Virus	Painful blisters	Antiviral drugs

Multiple Pregnancies:

• Identical twins: One zygote splits → same sex, identical genetics (~0.4%)
• Fraternal twins: Two eggs fertilised → different sexes possible, ~50% shared genetics
• Higher multiples: More rare; can be identical, fraternal, or mixed

Parturition (Childbirth):

Three stages:

1. Dilation: Uterine contractions → cervix dilates from 0 to 10 cm (~12 hours)
2. Expulsion: Baby moves through birth canal (~1 hour)
3. Afterbirth: Placenta delivered (~30 minutes)

Hormonal induction:

• Oxytocin (from posterior pituitary) → uterine contractions
• Prostaglandins → soften cervix
• Oestrogen/progesterone ratio changes (progesterone inhibits contractions throughout pregnancy)

⚡ WAEC Examination Patterns: Draw and label the human male and female reproductive systems. Explain the menstrual cycle and its hormonal control. Describe the process of fertilisation and early embryonic development. Explain the advantages and disadvantages of asexual vs sexual reproduction. Draw and label a flower and explain double fertilisation. Compare wind and insect pollination mechanisms.